Recently, efforts have been made to prepare heterophasic copolymers, such as an impact copolymer (ICP), using newly developed metallocene (MCN) catalysis technology to capitalize on the benefits such catalysts provide. Homopolymers prepared with such “single-site” catalysts often have a narrow molecular weight distribution (MWD), low extractables, and a variety of other favorable properties associated therewith, and copolymers often also have narrow composition distributions.
Unfortunately, common MCN, immobilized on a conventional support coated with an activator such as methylalumoxane (MAO), is not able to provide copolymer components with sufficiently high molecular weight and/or rubber loadings under commercially relevant process conditions. Compared to their Ziegler-Natta (ZN) system catalyzed counterparts, the iPP matrix of the ICP prepared using MCN has a low porosity, and is unable to hold a sufficiently high rubber content within the iPP matrix required for toughness and impact resistance. Also, the MCN-ICP has an MWD that is too narrow to obtain sufficient crystalline, low molecular weight polymer required for stiffness. The formation of rubber in a separate phase outside the matrix is undesirable, e.g., it can result in severe reactor fouling.
Pore structures in conventional iPP, whether from ZN or MCN systems, are understood to be generated from the fast crystallization of low molecular weight portions of the polymer that causes volumetric shrinkage during crystallization. Nello Pasquini (Ed.), Polypropylene Handbook, 2nd Edition, Hanser Publishers, Munich, pp. 78-89 (2005), reports volumetric shrinkage processes only generate low porosities for limited rubber loadings, e.g., 7% porosity from a conventional ZN catalyst system, and 16% more is obtained through the treatment of the MgCl2-supported ZN system via controlled dealcoholation, allowing the iPP matrix to be filled with a rubber content nearing 25 wt %. Cecchin, G. et al., Marcromol. Chem. Phys., Vol. 202, p. 1987, (2001), report that the micromorphologies of catalyst systems based on magnesium chloride-supported titanium tetrachloride (MgCl2/TiCl4) contribute to the morphology of the polymer granules. However, the rubber content of such an ICP obtained from an in-reactor, one-catalyst system is still significantly lower than the 40 wt % rubber content that can be achieved in a polymer blend ICP, which provides flexibility for the rubber content that is sometimes desired.
Accordingly, it has been elusive to balance the toughness and stiffness of a one-catalyst, sequential polymerization ICP, since on the one hand, the formation of high porosity and high fill rubber loading needed for toughness requires the presence of a high concentration of hydrogen to form the low molecular weight polymers needed for the fast-crystallization shrinkage, and on the other hand, polymerization under these conditions for maximizing porosity detracts from the stiffness of the resulting ICP.
U.S. Pat. No. 5,990,242 approaches this problem by using an ethylene/butene (or higher alpha-olefin) copolymer second component, rather than a propylene copolymer, prepared using a hafnocene type MCN. Such hafnium MCNs are generally useful for producing relatively higher molecular weight polymers; however, their activities are typically much lower than the more commonly used zirconocenes. In any event, the second component molecular weights and intrinsic viscosities are lower than desired for good impact strength.
WO 2004/092225 discloses MCN polymerization catalysts supported on silica having a 10-50 μm particle size (PS), 200-800 m2/g surface area, and 0.9 to 2.1 mL/g pore volume, and shows an example of a 97 μm PS, 643 m2/g surface area and 3.2 mL/g pore volume silica (p. 12, Table I, support E (MS3060)) used to obtain iPP (pp. 18-19, Tables V and VI, run 21).
EP 1 380 598 discloses certain MCN catalysts supported on silica having a 2-12 μm PS, 600-850 m2/g surface area, and 0.1 to 0.8 mL/g pore volume, and shows an example of silica having a 6.9 μm PS, 779 m2/g surface area and 0.23 mL/g pore volume (p. 25, Table 3, Ex. 16) to obtain polyethylene.
EP 1 541 598 discloses certain MCN catalysts supported on silica having a 2 to 20 μm particle size, 350-850 m2/g surface area, and 0.1 to 0.8 mL/g pore volume (p. 4, lines 15-35), and shows an example of a 10.5 μm particle size, 648 m2/g surface area and 0.51 mL/g pore volume silica (see p. 17, Example 12) for an ethylene polymerization.
EP 1 205 493 describes a 1126 m2/g specific surface area (SA) and 0.8 cc/g structural porous volume (small pores only) silica support used with an MCN catalyst for ethylene copolymerization (Examples 1, 6 and 7).
JP 2003073414 describes a 1 to 200 μm particle size (PS), 500 m2/g or more SA, and 0.2 to 4.0 mL/g pore volume (PV) silica, but shows examples of propylene polymerization with certain MCNs where the silica has particle sizes of 12 μm and 20 μm.
JP 2012214709 describes 1.0 to 4.0 μm PS, 260 to 1000 m2/g SA, and 0.5 to 1.4 mL/g PV silica used to polymerize propylene.
Other references of interest include US 2011/0034649; US 2011/0081817; Madri Smit et al., Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 43, pp. 2734-2748, (2005); and “Microspherical Silica Supports with High Pore Volume for Metallocene Catalysts,” Ron Shinamoto and Thomas J. Pullukat, presented at “Metallocenes Europe '97 Dusseldorf, Germany, Apr. 8-9, 1997.
Accordingly, there is need for new catalysts and/or processes that produce polypropylene materials that meet the needs for use in particular applications, such as one or more of: a good balance of stiffness and toughness, and/or other properties needed for high impact strength; homopolymers and copolymers with narrow MWD, low extractables, bimodal MWD, bimodal PSD, narrow composition distribution, and/or other benefits of MCN catalyzed homopolymers and copolymers; high porosity propylene polymers; heterophasic copolymers with a high fill loading of a second polymer component in a first polymer component; preparation of bimodal MWD or PSD heterophasic copolymers in a single-catalyst, sequential polymerization process; economic production using commercial-scale processes and conditions; and combinations thereof.